The beating rate of a heart is always changed. In other words, the interval between heartbeats is extended and shortened in a repetitive manner. This is the heart rate variability, and the change thereof is subjected to control of the autonomic nervous system. The autonomic nervous system is composed of a sympathetic nervous system which arouses and strains the mind and body, and a parasympathetic nervous system which sedates and rests the same. Various physiological and pharmacological researches beginning at the research by Akselrod et al. (American Journal of Physiology, 249: H867-875, 1985) have found that the component of 0.04 to 0.15 hertz (Low Frequency: LF) of the heart rate variability reflects both activities of the sympathetic nervous system and the parasympathetic nervous system, while the component of 0.15 to 0.4 hertz (High Frequency: HF) thereof reflects the activity of the parasympathetic nervous system. In addition, using the ratio of LF/HF as an index of the sympathetic nervous system activity is practiced by a number of researchers.
These indices are standardized by the European Society of Cardiology, and the like, and are used as a guideline (Malik, M. Heart rate variability: Standards of measurement, physiological interpretation, and clinical use. Circulation 93, 1043-1065, 1996), being utilized for pathology analysis of heart diseases.
In recent years, various attempts to utilize the heart rate variability for evaluation of the mental status have been made, and various methods for accurately and conveniently measuring the heart rate variability have been proposed. For example, in Japanese Patent Publication No. 4568825 (Patent Document 1), in consideration of the fact that the parasympathetic nervous system component of a heart rate variability is closely related to the breathing rhythm, a method for calculating the index of the parasympathetic nervous system activity on the basis of the length of one breath is proposed.
In Japanese Patent Publication No. 4487015 (Patent Document 2), in consideration of the fact that the LF and HF may greatly vary depending upon the individual difference or the difference produced in repetitive recording, it is proposed that a coefficient of variation of RR interval (CVRR) should be incorporated in the mathematical expression to indicate the sympathetic nervous system activity and the parasympathetic nervous system activity with new indices, i.e., (CVRR×LF/(LF+HF)) and (CVRR×HF/(LF+HF)), respectively, and in Japanese Unexamined Patent Publication No. 2008-229233 (Patent Document 3), a short-time Fourier method which uses heartbeat data obtained through as small as 100 beats for analysis is proposed.
In addition, a method for accurately analyzing the mental state through the analysis of the combination with or correlation to another index, such as a brain wave or a blood pressure, is also proposed. For example, in Japanese Patent Publication No. 4410234 (Patent Document 4) and Japanese Unexamined Patent Publication No. 2009-207763 (Patent Document 5), a method is disclosed which analyzes the correlation by determining the pattern of breathing, the rhythm of blood pressure change, the coefficient of R-wave amplitude in an electrocardiograph, and the like in addition to the heart rate variability.
However, while these methods disclosed in the above-mentioned patent documents contribute to the progress of the measuring or analyzing method, these patent documents have no specific disclosure about how to utilize the invention for analysis or diagnosis of the change in mental status or pathology of a psychiatric disorder. Thus, there has been no preferred method for implementing utilization of a heart rate variability index in analysis, diagnosis, or prevention of the change in pathology of a psychiatric disorder, however, there are available the following patent documents which intend implementation thereof, and many of them focus on evaluation of the stress state.
The change in heart rate variability in the stress state can be a change which is a healthy reaction per se, and therefore, the change is not the same as that related to a psychiatric disorder or the risk of onset thereof, however, there are no prior art documents which mention this point.
The invention disclosed in Japanese Patent Publication No. 4637963 (Patent Document 6) uses heart rate variability indices before and after deep breathing and from the difference, checks for health state. It is a publicly known fact that the rhythm of breathing can have an influence on the heart rate variability. The heart rate variability index for the parasympathetic nervous system depends on the rhythm of breathing. The health checking method using the rhythm of breathing is meaningful, however, it is supposed that the method perhaps focuses on the reaction persistence of the parasympathetic nervous system to the breathing. Also about depression, the document states that “it can be simply and instantaneously made clear that there is a high possibility of the subject having depression”, however, there have been no clinical observations proving that utilization of the heart rate variability due to the breathing load is useful for identification of depression. It is supposed that, for evaluation of the psychiatric symptom and the depression, a mental load should be imposed on the subject.
Further, the invention disclosed in Japanese Patent Publication No. 4238321 imposes a mental arithmetic stress on the subject, and from the change in heart rate variability power value and frequency, verifies the existence of a stress. However, in the patent document disclosing this invention, it is stated that “an average power value of 0.2 Hz or under is calculated”, but if an average is determined for this frequency band, the average involves both of the sympathetic nervous system component and the parasympathetic nervous system component, thereby the autonomic nervous system evaluation cannot be accurately performed, which is a problem.
Further, for the invention disclosed in Japanese Patent Publication No. 4238321, it is stated that “when the average power frequency is increased to or over a prescribed value with respect to the criterion value, it is judged that the subject is feeling an emotional stress”, however, there is no discussions about a fact that responding to a stressor is different from falling in a pathologic stress condition, which is a problem. In other words, it is often a healthy activity that the autonomic nervous system reacts to a mental load, while, in a pathologic stress condition, such reaction is not caused, is excessive, is delayed or disordered, but the invention is lacking consideration about this point.
Further, with the invention disclosed in Japanese Patent Publication No. 3250474, the heart rate and the indices of the sympathetic and parasympathetic nervous system activities are recorded in the resting state and the task performing state, the data being expressed as a distribution graph, and determines that, if the data for one state is overlapped on that for the other in many places, the subject has a stress.
However, this method gives indices of the sympathetic and parasympathetic nervous system activities on the basis of the dispersion of change in heart rate, such indices involving the activities of both sympathetic nervous system and parasympathetic nervous system, however, both of these activities have meanings in the contradictory directions, i.e., positive and negative ones concerning the mental status, thereby utilization of the indices having a mixture of those presents a problem. In addition, utilizing the unitary evaluation method, which is based on only the overlapping of distribution data in the resting state and that in the task performing state, for evaluation of a complicated mental status is difficult, as with the inventions mentioned above.
The heart rate fluctuation and other various autonomic nervous system activities are carried out under the control of the controlling center in the brain stem, however, it is known that they are heavily influenced by the mental status. The modern society gives much stress, and the incidence rate of the psychiatric disorder, such as depression, the number of employees on administrative leave and that of suicides are being increased. However, the diagnosis of the psychiatric symptom is subjective, and as described above, the early treatment and prevention using an objective index have not yet been put into practical use.